Sunday, February 14, 2010

Mini Lathe Modifications

I'm working on a gear reduction mod for my Harbor Freight Mini Lathe. I had three primary requirements for my design. First, that it was compact and altered the lathe as little as possible. Nearly as important, I wanted it to be cheap to build. Keeping it cheap meant using the original belt drive and pulleys, along with the gears from my Mini Mill left over after converting it to a belt drive.
I decided that I could meet these requirements by attaching the gear reduction directly to the motor.
This proved to be a bit of a challenge as it required every tool in my inventory: Lathe, Mill, Rotary Table, Index-able Boring Head. For material, I chose steel, specifically a discarded railroad track baseplate. It was 16mm thick in spots, perfect for my requirements.
The most difficult part was the ring that attaches to the motor. The back side is contoured to conform to the shape of the motor end bell precisely so that the stresses are not carried exclusively by the 4 small screws. I turned this 91mm diameter part on the mini lathe, which struggled with it. I'm hoping this mod makes turning such parts easier.
Then it needed a 22 mm hole bored for a bearing. The size and location of the hole is critical for proper gear mesh. In the top view of the motor ring, you will see some odd reliefs cut into it. These are to clear the bed casting. The inside has an odd shape to minimize blockage of the vents on the motor.

Also difficult was the arm which supports the other bearing and idler pulley. This had to align perfectly to avoid bearing binding, and the location of the idler pulley is critical. The idler is necessary because the belt would rub on the lathe housing without it, for the motor belt pulley is now moved 32mm towards the rear from it's original position. The idler also provides an additional benefit by keeping the belt in contact with the motor pulley for more than 190 degrees around the pulley. This may help it withstand the additional torque.
The idler location is critical because of the limited range for belt adjustment. Previously, I converted my 7x10 to a 7x14 using LMS's kit. One big advantage of the conversion that I hadn't realized at the time is that it has room for a larger motor. It also has a better motor mount. While these features made my gear reducer much easier to install, the idler location is still very important.

The idler pulley is made from a pair of 8mm x 22 mm bearings, same as used for the intermediate shaft. The intermediate shaft has 3 diameters: 8 mm for the bearings, 9 mm for the belt pulley, and 10 mm for the mini mill gear. Why did I turn the shaft ends to 8mm instead of using 9mm bearings? Because the larger 24mm dia of 9mm bearings are too big for my design.
Why is my belt pulley 9mm instead of the usual mini lathe 8mm? Because I took advantage of the 7x14's extra space to use a milling machine motor, which has a 9mm shaft.  I bought a pulley from LMS before I designed the gear reducer.  That is one problem that others who may attempt this mod will have to contend with. The mini mill motor gear I'm using has a 9mm hole. I'm not sure if such a gear is available in 8mm.

Sorry I don't have drawings at this time. While I made preliminary drawings, much of the final design was tweaked as I test fitted it. I do not yet have any performance info to share, as I'm waiting on parts to modify the motor controller to handle the larger motor. The 250 watt Mini Lathe and 350 watt Mini Mill controllers appear identical except for the values of the current limiting resistors and the size of the heat sink.
Overall, I'm satisfied with the design (assuming that there is some noticeable performance gain) The benefits will be hard to measure precisely, as I'm simultaneously upgrading to a Mini Mill motor, which is probably good for a 50% increase in torque alone. The gears provide a 2.2 to 1 reduction. Combining the two, I'm hoping that the torque increase is significant. I had difficulty turning the 91 mm ring at an appropriately slow speed without stalling. That forced me to turn it too fast, resulting in the poor finish on the back side.
Below:  Motor installed.  Idler arm is then installed.  Changing belts is easy, simply remove idler arm.
With idler arm installed: While the top of the idler arm appears to interfere with the headstock, it does not. 
Last picture:  With gear cover installed.  I had hoped I would not need to trim the cover, but it was necessary. However, the arm could be redesigned to minimize the amount of trimming needed.

The motor cover needs an extension added. However, since I changed to a larger motor, I need a much larger cover and will make one from scratch.
UPDATE: The installation is complete, and I'm extremely pleased with the results. The lathe will turn as slow as 15 RPM, and at only 20 RPM I can't stall it with my hand. I can't even slow it down.

UPDATE:  I began turning another motor ring for a second gear reducer.  Like the first, it is railroad track steel.  But this time, my lathe was ready for it. At only 30 RPM, it tore into that steel with a vengeance, never slowing down, and throwing off industrial size chips.  Soon the cutting oil began to smoke, so I reduced the feed. But like the Energizer Bunny, the lathe wanted to keep going.
This piece is 4 inches in diameter, and thanks to the slow speeds and heavy cut, there was zero chatter. An impossible feat on the unmodified lathe.

The plastic drive belt pulley lasted a while, but the stress of cutting large diameter parts snapped the hub at the key slot.   I had a replacement on hand. but that one lasted only a few hours.  With everything else beefed up, this pulley is now the weak link, so I made a steel hub for it.  Hopefully this lasts a while because the next step would be an all steel pulley.

I have a supply of motor mounts available for mini lathes where the motor is bolted to two slots in the front of the lathe bed.  These mounts move the motor 10mm towards the tailstock, and rotate the motor about 10 degrees.  The rotation moves the motor end bell screws out of the way of the gear reducer.  Two flathead M6 screws are included.  Price is $10, which includes Priority Mail shipping.
Contact robertsprojectsmail"at"


  1. Nice solution! As the motor-heads say: "There ain't no replacement for displacement"!

    =Alan R.

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